Recently, great demands for lowering the cost of the optical module have been made in order to introduce the optical fiber communication system into the subscriber network system on a full scale. From a viewpoint of this, with respect to the packaging technology, a resin sealing method (a plastic package method) is influential in place of a hermetic sealing method for a metallized package or a ceramic package which has heretofore occupied the mainstream in the field of optical devices. This is because the resin sealing method is suitable for mass- and inexpensive production.
In the past, in the field of general semiconductor devices such as LSI and hybrid IC, the following various methods have been known as resin sealing methods. (1) a transfer molding method (a method for compressing preheated powdery resin to transfer it to a mold), (2) a casting injection method (method for injecting a liquid resin into a mold frame), (3) a potting injection method (a method for injecting a liquid resin into a resin case), (4) a cap sealing method (a method for adhering a cap to a hollow case with resin), (5) a dipping method (a method for dipping an element into a resin liquid vessel), and (6) a dropping method (a method for dropping a liquid resin onto an element).
Further, the following examples in which the aforementioned resin sealing methods are applied to the optical module have been known. For example, there are Japanese Patent Application Laid-Open No. 2-271308 (Article 1), Japanese Patent Application Laid-Open No. 1-304405 (Article 2), Japanese Patent Application Laid-Open No. 7-321407 (Article 3), Proceedings of the Conference of the Institute of Electronics, Information and Communication Engineers in 1996, Separate Volume--Electronics 1, pages 478 to 479 (Article 4), the same collection, page 216 (Article 5), and the same collection, page 207 (Article 6).
Article 1: In the receptacle type optical module for fiber optic transmission, a hermetic seal by way of a metallized package and a resin seal by way of transfer molding are jointly used. A laser diode and a photodiode are once hermetically sealed in a can type metallized package, and the can type metallized package is molded by the transfer molding method together with electronic circuit parts and a lead frame.
Article 2: In the receptacle type module, a potting injection type method is used. A resin case comprises a black epoxy resin molded article. A cylindrical receptacle is provided in part thereof. A light emitting diode and a light receiving diode are installed on a lead frame, and are received in the resin case. The resin case are filled with transparent epoxy resins by potting. The light emitting diode or the light receiving diode is directly sealed by the transparent resin.
Article 3: This is concerned with a pickup light source of an optical disk. In this example, a casting injection method (or a transfer molding method) and a resin sealing method by way of a dropping method are jointly used. The laser diode is fixedly secured to the lead frame through a heat sink, and these are sealed by casting of transparent epoxy resins. A silicone resin is coated on the surface of the laser diode by the dropping method in order to prevent the optical damage of the sealed resin caused by emitting light of laser and the peeling of an interface between the laser diode and the sealed resin.
Article 4: In the pigtail type module for fiber optic transmission, a casting injection method is employed. In this example, the laser diode and the bare portion of the pigtail fiber are sealed with resin. That is, the laser diode and the extreme end of the bare fiber are secured to the heat sink, and the heat sink is fixedly secured to the metallic stem. They are molded by casting of transparent epoxy resins. The jacket portion of the pigtail fiber is not sealed with resin.
Article 5: In the pigtail type module, a hollow resin case is closed by a cap sealing method. The laser diode and the bare portion of the pigtail fibers are secured to a silicon base plate, and the base plate and the jacket portion are secured to the resin case. A lead frame is insert-molded in the resin case. The bare fiber and the base plate, the jacket and the resin case, and the cap and the resin case are bonded one another by ultraviolet setting resins.
Article 6: In the receptacle type module, the cap sealing method and the transfer molding method are jointly used. A laser diode, the extreme end of a ferrule with fiber, and a lead frame are secured to a silicon substrate to constitute a substrate assembly. The laser diode and the ferrule are once sealed by adhering a silicon cap on the substrate with the epoxy resin, and the substrate assembly is sealed by the transfer molding of black epoxy resins except the rear end of the ferrule and the outer lead portion of the lead frame. The receptacle is constituted by a part of the transfer molded article, the rear end of the ferrule and a housing provided by other parts.
The plastic package is influential for lowering the cost of the optical module as compared with the metallized package or the ceramic package, but has difficulties such that generally, the moisture permeability is high, and the coefficient of thermal expansion is large. These factors synthetically lower the reliability of the plastic package. It is therefore important, for putting the plastic module in practical use, how to secure the reliability while making use of the merit of lowering the cost of the plastic package.
It is concretely necessary to make consideration relative to the aforementioned problems from the following two aspects. First, the moisture resistance should be increased in respect of constituent parts of the module such as an optical device, an optical fiber, a base plate and so on. Secondly, with respect to the optical coupling between the optical device and the optical fiber, it is necessary to suppress extremely highly a deviation in position caused by the thermal stress, the external force, the molding pressure, etc. In particular, in the semiconductor laser module, a demand relative to such a position is severe. This is because the spot size of the laser diode is smaller than the spot size of the light emitting diode and the light receiving diameter of the photodiode.
Any of the aforementioned prior arts have both merits and demerits with respect to such demands as noted above, and fail to respond to the demands of the current industrial world. The present invention is a new invention in connection with these various matters. The aforementioned problems with respect to the prior arts will be briefly mentioned in order to better understand the background of the present invention.
Article 1: The number of parts and the number of assembly steps so increase as not to make a good use of the plastic package, i.e., the low cost. That is why the metallized package and the plastic package are jointly used.
Article 2: The cylindrical receptacle is formed in a part of the resin case. Considering the dimensional accuracy of the receptacle itself and the deformation of the receptacle caused by the insert- and pull force of connector and the thermal expansion, the module in Article 2 can be applied to a light emitting diode having a large spot size but is not suitable for a laser diode having a small spot size.
Article 3: A light source for an optical disk is concerned, which is not used for transmission of an optical fiber as in the present invention. Naturally, consideration of the seal of fiber and the reliance of optical coupling is not taken in Article 3.
Article 4: Only the bare portion of the pigtail fiber is sealed with resin, and the jacket portion is exposed to outside. Accordingly, the thermal stress and the external force are concentrated on the interface between the bare fiber portion and the jacket portion to bring forth a problem in that the bare fiber is broken. There is a difficulty that since when the casting is performed, voids tend to be mixed, water moves in from the channel to corrode optical ends of optical devices and electrodes.
Article 5: While the pigtail fiber is received in the hollow resin case, reference is not made to the thermal stress and moisture resistance of the resin case. Further, reference is not made to materials for the resin case and molding method. In this technique, various problems occurs unless various items mentioned above are sufficiently selected. That is, due to a difference in the coefficient of thermal expansion between the bare fiber (quartz glass) and the resin case, the bare fiber protrudes from the jacket, and the crack occurs in the bare fiber due to the concentration of stress. Further, vapor permeated into the hollow case becomes dewed on the surface of the optical device or the bare fiber to give rise to the corrosion of the optical device, or to progress the growth of crack, resulting in the rupture of the bare fiber.
Article 6: The optical device and the ferrule with fiber are sandwiched between the silicon substrate and the silicone cap, and sealed. However, the elastic modulus of the resin package member and the anisotropy of the thermal coefficient of expansion of the resin package member are not taken into consideration.
As briefly mentioned above, the prior arts lack some consideration in relation to the number of parts, thermal stress and moisture resistance concerned with the cost. The present invention provides an optical module compatible with the lower cost and the higher reliance by the plastic packaging technique.